All platforms running Thunderbird: If you love Apple Mail's look and feel but not Mail.app's Mac-onlyness, you can dress Thunderbird up to look just like it.

The Mail.app theme for Thunderbird turns its interface into a good-looking Mac UI. I haven't done much with Thunderbird themes (or Firefox, for that matter) in the past, but this Mail.app theme does look hot on my PC. Got a favorite Mozilla product theme? Tell us about it in the comments. The Mail.app T-bird theme is a free download that works with and wherever Thunderbird does. — Gina Trapani